During the 1950s and 1960s major efforts were made to transmit human hepatitis to laboratory animals. These studies were complicated by difficulty in obtaining inocula of known infectivity, the absence of specific diagnostic tests for hepatitis A and B, and the relatively insensitive assays which were available for detecting hepatocellular damage. In some studies only evidence of clinical disease was sought, while in others, workers used bilirubin levels, cephalin-cholesterol flocculation tests or bromsulfthalein excretion tests as markers of infection. During a study of the susceptibility of marmosets to human viruses, several animals were inoculated with serum or plasma which had been collected from patients with hepatitis during the acute phase of their illness. Because of the availability of specific assays for anti-HAV, it became possible to examine a wide range of nonhuman primates in zoos, as well as recently captured animals, and animals held in breeding colonies for evidence of past infection with HAV.